Pilots of modern aircraft, in particular of fighter planes, are supplied with respiratory gas by systems which deliver pure oxygen or an oxygen-enriched mixture.
A control valve of the proportional type with one inlet and two outlets for an onboard system for delivering respiratory gas is described in EP-A-0 499 505 (Zapata et al.).
In current practice, an emergency oxygen source is provided as standard in order to deal with possible failure of the main oxygen source, and a selector element, activated automatically or manually, allows the user to switch the inlet of the control valve to the main source or, if this fails, to the emergency source.